As schematically shown in FIG. 1, the Faraday effect is the rotation of the plane of polarization that occurs when a beam of light whose plane is polarized in the y direction passes through a certain optical material (having a length l) in a direction parallel to the lines of force of a magnetic field H. By this effect, the plane of rotation of the light is rotated by an angle .theta. which is proportional to H and l and represented by EQU .theta.=VelH (1)
wherein Ve is the Verdet constant.
As shown in FIG. 2, a beam of light from a light source 6 is linearly polarized by a polarizer 7 and is passed through an optical material 1 and an analyzer 8 that sense a change in the intensity of light due to the change in the direction of plane polarization, and subsequently, the detected change in light intensity is converted to an electrical signal V.sub.out by a photo detector 9. If the polarizer is positioned at an angle of 45.degree. with respect to the analyzer 8, the output of the resulting electrical signal V.sub.out is indicated by the formula: EQU V.sub.out =1/2K(1+sin 2.theta.) (2)
wherein K is a constant determined by the intensity of incident light and the sensitivity of the photo detector 9. The increment in V.sub.out due to the application of a magnetic field is represented by EQU V.sub.out =1/2K sin 2.theta..congruent.1/2K.multidot.2.theta.=KVelH(3)
and this formula shows that the resulting output is proportional to the magnetic field strength H. It is well known that by placing the device of FIG. 2 under a live conductor 10, the intensity of the induced magnetic field or the current that has induced it can be measured.
Lead glass has conventionally been used as a material that exhibits the Faraday effect (such material is hereinafter sometimes referred to as a Faraday material) to enable the measurement of an electrical current or magnetic field strength. This is mainly because lead glass has a relatively great Verdet constant (0.098 min/0e.cm at a wavelength of 633 nm) and an inherently good temperature stability which is characteristic of diamagnetic glass. The Verdet constant of glass can be doubled by adding a metallic element (e.g. Tb), but the resulting glass is paramagnetic and its Verdet constant is an inverse function of the absolute temperature T.
Bismuth silicon oxide (Bi.sub.12 SiO.sub.20 and hereinafter abbreviated as BSO), or bismuth germanium oxide (Bi.sub.12 GeO.sub.20 and abbreviated as BGO), are Faraday materials that have a higher sensitivity than lead glass and which exhibit good temperature stability. They are effective in providing a magneto-optical converter having better characteristics.
In measurement of an electrical current using one of the Faraday materials listed above, the intensity of the induced magnetic field can be increased by confining it within a small space. The operating theory of this method is illustrated in FIG. 3, wherein a Faraday element 12 is positioned between the opposite ends of an annular iron core 11 around a conductor 10 and a beam of light from a light source is guided through an optical fiber 13, modulated by the magnetic field and thereafter directed to a photo detector through an optical fiber 14. Theoretically, this method is capable of measuring a current ranging from a few amperes to several thousand amperes, but since the magnetic field strength is decreased with the increasing distance between the opposing ends, most of the magnetooptical converters currently used are designed to intensify the magnetic field by minimizing the distance between the opposing ends of the iron core and at the same time cause the beam of light to travel within the Faraday material many times so as to increase the distance over which the magnetic field acts on the beam of light.
A typical arrangement conventionally used to achieve this object is depicted in FIG. 4. A beam of light from a light source is guided through an optical fiber 13 into a Faraday material 1 provided with reflecting layers 15 which is positioned between the opposing ends of an iron core 11. The plane of the light is linearly polarized by a polarizer 7 and is reflected many times within the Faraday material, the polarized light is passed through an analyzer 8 that detects the direction of polarization forming a 45.degree. angle with respect to that of the polarization by the polarizer. The light is thereafter sent to a photo detector through an optical fiber 14. However, this arrangement has the following defects:
(1) The light does not travel over the entire distance between the opposing ends of the iron core, so the necessary sensitivity cannot be achieved without increasing the number of reflections occurring in the Faraday material; PA0 (2) To increase the number of reflections the light must be introduced at an angle with respect to the surface of the Faraday material, and the desired amount of light picked up at the other end. Therefore, an exact adjustment of the angle of incidence is necessary but this is difficult and its difficulty is a function of the number of reflections.